Until now, use has mainly been made of integrating detectors for detecting gamma and X-ray radiation, which detectors generate an electrical signal proportional to the intensity of incident X-ray radiation over a given time interval. In the process, the detectors are structured into individual sensors for spatially-resolved acquisition of the radiation. By way of example, such detectors are used in computed tomography systems. In such systems, X-ray projections of an examination region of a patient are acquired from many different projection directions by way of the detector, and slice or 3D images are calculated therefrom according to known reconstruction methods.
A development of these systems resides in the use of so-called counting detectors. These allow separate acquisition of absorption events for each individual radiation quantum. The counting and energy-selective acquisition more particularly allows the evaluation of material-specific properties of the examination region with, at the same time, a reduced X-ray dose; this can be converted into increased contrast in the reconstructed image.
Use can be made of both direct-conversion and optically counting detectors. In the case of direct-conversion detectors, the X-ray quanta are directly converted into free charge carriers in a semiconductor layer as a result of interaction processes with a semiconductor material, which charge carriers are accelerated in an electric field between two electrodes arranged opposite one another. CdTe, CdZnTe, CdTeSe or CdZnTeSe semiconductor materials can for example be used as semiconductor material. The charge carrier transport induces currents on the electrodes, which can be registered as an electrical pulse. By contrast, in the case of optically converting detectors, the conversion occurs in two stages. In a first stage the X-ray quantum is converted into light pulses by means of a scintillator with a short decay time, for example by means of a BGO, LSO, or CuI scintillator. The light signal thus generated is converted into an electrical pulse in a second stage in a photodiode that is optically coupled to the scintillator. In both cases, the electrical pulse thus generated has a pulse height that is characteristic for the energy of the incident X-ray quantum.
The pulses are subsequently routed to an evaluation unit for subsequent processing. There, for each sensor, a count value is generated for at least one energy threshold, which count value represents the number of X-ray quanta above the respective energy threshold. In the process, the count is performed by means of a count circuit (trigger circuit), which for a generated pulse increments the count value for the energy threshold when a pulse threshold corresponding to the respective energy threshold is exceeded. In the case of simple, conventional CT imaging only one energy threshold is required and it typically lies in a range between 15 keV and 35 keV. A further threshold, for example in the range between 50 keV and 80 keV, is provided for dual-energy imaging or optimized conventional CT imaging.
However, count values can be influenced by absorption events of X-ray quanta in adjacently arranged sensors and hence they can be falsified. By way of example, in the case of a so-called K-escape effect, part of the energy of an X-ray quantum is carried into the neighboring sensor by fluorescence effects. Moreover, if an X-ray quantum is converted in the vicinity of the sensor edge, the resulting signal may be distributed to locally adjacent sensors. This effect is also referred to as charge sharing or, even better, energy splitting. Both effects often result in double counting of individual X-ray quanta if the energy threshold is low compared to the energy of the primary X-ray quantum. Secondly, count values in respect of relatively high energy thresholds, which are exceeded by the energy of the primary X-ray quantum in the case of complete conversion, are underestimated.
In the field of human medicine, one is moreover confronted with relatively high quantum flux rates of e.g. more than 108 X-ray quanta/mm2*s. Separation of X-ray quanta, which are successively incident over time, on the basis of the registered signals can therefore only be ensured if the sensor areas are reduced in size, for example to less than 0.1 mm2. However, in most converter materials this is connected with a significant increase in the K-escape effect and energy splitting. Double counting of individual X-ray quanta at lower energy thresholds and the loss of count events at correspondingly higher energy thresholds have a negative effect on the image contrast and the image noise.
Hence, it is an object of the present invention to develop a counting detector and a computed tomography system with such a detector such that a negative effect on the image quality resulting from double counting of individual X-ray quanta is at least markedly reduced.